SENSE OF SIGHT. 435 



sufficient to make the luminous rays pass through a lens cut 

 in such a manner as to restore the equilibrium between the 

 two meridians, so that the rays, after passing through this 

 lens and through the medium of the cornea, follow the same 

 direction as the rays which pass through an ordinary cornea. 

 The surface of the glasses used for this purpose is cylindrical, 

 instead of being spherical, and they are arranged in such a 

 manner that the convergence which they produce on a single 

 plane corresponds exactly with the plane of that meridian 

 according to which the surface of the cornea of the eye is in- 

 sufficiently convex : in this way, such want of convexity is 

 remedied. 



II. Enveloping membranes of the eye. 



Beginning from the outside, and going towards the centre 

 of the eye, we find that there are three envelopes of the eye, 

 the white of the eye, the choroid, and the retina ; the latter 

 being the membrane which is especially endowed with sensi- 

 bility. We will consider the two first as protecting envel- 

 opes, intended to assist, and even to modify the functions of 

 the other and essential portions of the eye. 



1. The Sclerotic. The sclerotic coat of the eye forms, 

 as it were, its skeleton. This membrane is intended to pre- 

 serve the form of the ocular globe, and into it the muscles 

 that move the globe are inserted. It has a fibrous texture in 

 man, but cartilaginous and even bony in birds and reptiles. 

 This white of the eye in front undergoes a change ; from 

 being white and opaque, it becomes transparent and color- 

 less, constituting the cornea, which we have already studied. 

 The cornea is more convex, and belongs to the segment of a 

 sphere whose radius is shorter than that of the white of the 

 eye, or, in other words, of that of the other portions of the 

 eyeball (Fig. Ill, p. 491). 



2. The Choroid Tunic or Coat. The choroid coat lines 

 the sclerotic throughout, except where it joins the cornea, 



and enters the anterior chamber of the eye, where, in front 

 of the crystalline lens, it forms a diaphragm called the iris. 

 We have, therefore, to study both the choroid coat, properly 

 so called, and the iris. 



A. The choroid, properly so called, is essentially a vascular 

 membrane ; its inner surface is also lined with a layer of 

 pigment cells, of a regularly hexagonal shape ; it contains, 

 finally, muscular elements, especially in front. Three princi- 

 pal functions are, therefore, assigned to this membrane. 



1 



